Early in the spring (after a few months of browsing online) I finally found a set of 20'' detroits for a reasonable price on ebay. The downside was some damage was present, including slightly dinged paint (jag MDZ silver touch up paint was a match), curbed trim rings (see below), and a bend on the inside of one rim from a pothole (left to a professional wheel repair shop, ~$200 to repair & balance). A few tips for anyone looking for a set of 20'' BBS's:

Have patience & watch ebay!
20'' BBS's do pop up from time to time. Sometimes the prices are out there, but there are deals on nice condition sets for ~ $2k. Also ebay UK has auctions from time to time, I was actually close to purchasing a set from a gentleman in the UK, and shipping would have been very reasonable, around $300. As with almost anything on ebay, expect them to be in worse conditions that the pictures!

Replacing Trim rings:
This is definitely a DIY job. I ordered two replacement rings from ebay uk, I heard horror stories of taking the two piece wheels apart but I had absolutely no issues. You will need 12 point star bolts, which can be a bit tricky to find. Other than that, a breaker bar, and applying heat to each Ti bolt before removing will make the change as smooth as butter. All old adhesive should be removed for new trim ring to fit properly, then just re-assemble. During re-assembly, the hollow "air" bolt should have a bit of silicone added to it. Word of caution, watch out for the trim ring edges, they're sharp!

Cheap DIY fix for slightly curbed trim rings:
One of the other wheels had slight curb rash on the chrome trim ring, which was easily remedied by a metal file, sand blocks, and gradually smoother sandpaper finished with metal polish. The rings have enough material where slight curb rash can easily be fixed like this, without changing the ring (and without disassembling the wheel). As a rule of thumb I'd say if you can see the ring is deformed (breaks light noticeably), there's no fixing it.

Paint the disc hub/calipers before installing your new wheels!
The 20'' wheels really make your breaks pop. Rust under there looks awful, with a bit of time you can paint your calipers (Silver, Red, Black, consider a JAG high heat vinyl decal!), and some silver high heat rust paint on your disc hubs will really clean up the look.

Photos!:

BEFORE:



AFTER:

 

I've been thinking about doing the same! What a GREAT appearance.

 

Great write-up and a superb result.
Graham

 

Fantastic! Detroit wheels are my favorites. Maybe some day…

Then I'd probably need to paint my calipers too.

 

I have the Detroit wheels but quite honestly I prefer your original wheels!

I think there is just too much open space with the Detroit.

Now you have all that viewing space you definitely need the big discs and Brembo's to complete the set up!

Then what do I know, I prefer the original amber side indicators/reflectors etc. Especially with my Slate Grey paint.

 

I would love to trade my stock 18's for a set of those!